1. Field of the Invention
The present invention relates to apparatus for determining the rotational position of a motor, and more particularly to apparatus of the type which determines the rotational position of a DC permanent magnet motor by counting commutation pulses.
2. History of the Prior Art
It is frequently necessary to track the position of a motor in feedback, servo or similar systems. In automotive air conditioning systems, for example, a DC permanent magnet motor is typically used to drive a ventilation door. The position of the ventilation door is determined by tracking the position of the drive motor.
An early approach to tracking motor position involved the use of mechanical components such as potentiometers built into an attenuator gear train. A window comparator was activated to force the desired actuator position. Such mechanical systems were subject to inaccuracies, wear and premature failure.
More recently, motor position has,been tracked by detecting and counting commutation pulses. In the case of a DC permanent magnet motor, as the motor rotates, the current is periodically interrupted due to the action of the motor brushes. This produces a steep pulse or spike in the motor current. Such commutation pulses are detected and counted using a microprocessor to provide an indication of motor position. The pulses have a very short duration, typically about 50 nanoseconds, so as to be distinguishable from other pulses and variations in motor current through the use of filtering techniques.
Arrangements for tracking motor position by counting commutation pulses typically include circuitry for detecting the pulses and then conditioning the pulses such as by amplification and filtering so that valid commutation pulses can be detected to the exclusion of noise and other unwanted signals. In one particular prior art circuit, for example, the motor is coupled through an inverter to convert the negative pulses into positive pulses before application to a filter and amplifier for high pass filtering and amplification. The output of the filter and amplifier is a series of pulses which are counted by a pulse counter such as a microprocessor. The motor is coupled to be driven by a motor drive circuit, which is coupled to ground through a sense resistor.
Pulse counting circuits of the type described have a number of disadvantages. One problem occurs during motor braking. In that mode, the motor current tends to circulate through the motor drive circuit to the exclusion of the sense resistor, so that commutation pulses may not be detected. Also, the filtering and amplification process is such that the high gain amplification of detected pulses frequently results in false detection. A major problem in such systems is to distinguish between valid motor commutation pulses and noise or lower amplitude pulses common with aging motors. The circuit must be capable of detecting commutation pulses of varying shapes and amplitudes, to the exclusion of noise and other unwanted pulses which may not differ substantially from the commutation pulses. Variations in the commutation pulses tend to increase as the motor ages, making the accurate detection of the commutation pulses even more difficult. A further problem relates to the fact that such pulse detection and conditioning circuits are only capable of detecting pulses of one polarity. For bi-directional motor operation, two such circuits are needed in order to sense the pulses of opposite polarity. The circuits themselves are relatively inefficient, and typically require a large number of components to implement.
It would therefore be desirable to provide an improved arrangement for tracking motor position. It would furthermore be desirable to provide an improved motor tracking arrangement which provides for the accurate detection of commutation pulses during motor braking as well as at other times when noise and other unwanted signals are present. It would still further be desirable to provide a motor tracking circuit capable of sensing pulses of opposite polarity for bi-directional motor operation.